1. Field of the Invention
The present invention relates to rotary tools, such as disk grinders.
2. Description of the Related Art
In general, this kind of rotary tools is configured such that rotation of an electric motor disposed within a tool body is transmitted to a spindle via a reduction gear train that reduces the rotation of the electric motor. In the case of disk grinders, the reduction gear train includes a drive-side bevel gear and a driven-side bevel gear meshing with the drive-side bevel gear. The spindle has a circular grinding wheel mounted thereon and is supported so as to be rotatable about an axis that is perpendicular to the output shaft of the electric motor.
Due to a suitable backlash provided in the reduction gear train or due to the other factors, a start shock may be produced at the time when the gears are engaged for transmitting torque as the motor is started. Therefore, in this kind of rotary tools, there have been proposed various techniques for resolving or reducing the start shock.
For example, Japanese Laid-Open Patent Publication Nos. 2002-264031 and 2010-179436 disclose techniques relating to impact attenuation mechanisms for reducing a shock that may be produced as a motor is started. The impact attenuation mechanisms include a radially resiliently deformable C-shaped torque transmission member that is interposed between a driven gear and a spindle in a torque transmission path. As the motor is started, the torque transmission member resiliently deforms in the radial direction, so that a start shock is absorbed while the drive torque is transmitted from the driven-gear to the spindle via the torque transmission member.
However, because the drive torque is necessary to be transmitted via the C-shaped torque transmission member in the above known impact attenuation mechanisms, the driven gear is necessary to be rotatably supported on the spindle. To this end, the spindle is inserted into a support hole (an inner circumferential hole) formed in the driven gear while a suitable clearance is provided between the inner circumferential surface of the support hole and the outer circumferential surface of the spindle. The clearance is set, for example, to be between about 0.004 mm and about 0.050 mm in order to minimize the displacement (offset with respect to the center) between the driven gear and the spindle, while ensuring easy assembling of these elements. Because the driven gear is rotatably supported on the spindle while a small clearance is provided therebetween, it may be possible that the inner circumferential surface of the support hole of the driven gear and the outer circumferential surface of the spindle are worn through contact therebetween. If the wear progresses, the gear and the spindle may be displaced with respect to the center from each other to cause improper meshing of the gears, resulting in generation of vibrations. As a result, the durability of the electric motor may be lowered.
Therefore, there has been a need in the art for a rotary tool that has an impact attenuation mechanism provided between a driven member and a spindle and that can reduce wear of the driven member and the spindle.